518 
On Water and Air. 
[August, 
appears to be floating upon a kind of oil, and that is due 
undoubtedly to the air beneath ; and it was supposed, even 
by Whitworth himself, that when you press down this plate 
you by so doing squeezed out the air, and that this one was 
firmly clasped to the other, and that this firm clasping of the 
under plate to the upper plate was due to the air being 
squeezed out.- There are many who thought that this was 
not the case, and that it was a case of firm molecular cohe- 
sion. To experimentally test this question the following 
arrangement was made -.—Here are two Whitworth planes, 
p, p (Fig. 40) ; they are placed under the receiver of an air- 
pump. The planes have been pressed together, and are 
firmly adhering. The under plane has attached to it a 
weight of lead weighing 20 lbs. The upper plane is attached 
to a rod which slides air-tight through the upper part of the 
receiver, and by means of the handle H I am able to lift the 
